The present invention relates to a fuser apparatus useful for heat-fixing a heat-softenable toner material to a substrate and, more particularly, to a roller assembly containing an externally heated fuser roller whose outer layer includes a cured fluorocarbon random copolymer.
Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. In such methods, the toner is then generally transferred to a surface of another substrate, such as, for example, a receiver sheet comprising paper or a transparent film, where i fixed in place to yield the final desired toner image.
When heat-softenable toners comprising, for example, thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to soften the toner that has been transferred to the receiver sheet surface, then allowing or causing the toner to cool.
One well-known fusing method entails passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, a heated roller, usually referred to as a fuser roller, that contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roller, usually referred to as a pressure roller, serves to press the receiver sheet into contact with the fuser roller. In some other fusing methods, the configuration is varied, with a flat plate or belt replacing the fuser roller and/or pressure roller. The description herein, while generally directed to a generally cylindrical fuser roller in combination with a generally cylindrical pressure roller, is not limited to fusing systems having members with those configurations. For that reason, the terms xe2x80x9cfuser memberxe2x80x9d and xe2x80x9cpressure memberxe2x80x9d are generally used herein in place of xe2x80x9cfuser rollerxe2x80x9d and xe2x80x9cpressure rollerxe2x80x9d.
The fuser member usually comprises a rigid core covered with a resilient material, which will be referred to herein as a xe2x80x9cbase cushion layer.xe2x80x9d The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness, often expressed as xe2x80x9cstorage modulusxe2x80x9d, and the stability of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it was found advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example, in a fusing system having a pressure roller and a fuser roller, by slightly modifying the shape of the pressure roller. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rollers, can be established by, for example, continuously varying the overall diameter of the pressure roller along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, resulting in the pressure roller having a xe2x80x9cbow tiexe2x80x9d or xe2x80x9chourglassxe2x80x9d shape. This shape causes the pair of rollers to exert more pressure on the receiver sheet in the nip in the areas near the ends than in the vicinity of the roller midpoints. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roller begins to permanently deform to conform to the shape of the pressure roller, and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation, often referred to as xe2x80x9ccreepxe2x80x9d, of the base cushion layer of the fuser roller is the greatest contributor to this problem.
U.S. Pat. No. 4,372,246 discloses an externally heated fusing member whose outer layer is formed from a silicone elastomer containing dispersed iron oxide particles. In the illustrative examples, the fuser rolls were heated to a maximum surface temperature of only 270xc2x0 F. (132xc2x0 C.).
U.S. Pat. No. 5,208,638 discloses a perfluoroelastomer containing a dispersion of conductive material for use as an intermediate transfer surface in an electrostatic image transfer system employing liquid toners.
U.S. Pat. No. 6,061,545 discloses a heat roller that includes an internal heating element and an outer layer of fluoropolymer containing particles of a thermally conductive filler. The listing of suitable fluoropolymers in the reference includes PTFE, FEP, PFA TEFLON(copyright), VITON(copyright), and FLUOREL(copyright) materials.
U.S. Pat. Nos. 6,127,041 and 5,017,432 are both related to fuser members. In U.S. Pat. No. 6,127,041, the disclosed fuser member has a metallic core on which is coated a composite layer comprising a silicone T-resin, a crosslinked poly(dialkylsiloxane) incorporating an oxide, and a silane crosslinking agent. The oxide in the composite layer of the fuser member can be an oxide or a mixture of oxides, aluminum oxide, iron oxide, tin oxide, zinc oxide, copper oxide, nickel oxide, and silica being listed in the reference as typical oxides. In U.S. Pat. No. 5,017,432, the disclosed fuser member has a fusing surface that comprises VITON GF(copyright), poly(vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene), that has been cured with a nucleophilic curing agent.
External heater rollers for nip-forming rollers such as the DigiSource 911(trademark) fusing apparatus are internally heated. These types of rollers usually have either an anodized surface layer or a TEFLON(copyright) surface layer with very low thermal resistance. The thinness of these layers does not allow a large contact length when a nip is formed with a fuser roller. A longer nip would allow more heating time for the fusing surface. To achieve this, an elastomer layer thicker than the anodized or TEFLON(copyright) surface layer could be applied to the heater roller. However, it would create a time delay for the heat energy to reach the heated roller surface due to the increase in thermal resistance that results from increased thickness of the elastomer layer. This time delay would increase thermal response time when altering the fuser roller temperature for any process reason and could preclude the use of gloss control through fuser roller temperature changes. Various receiver types have different thermal properties that affect gloss and fusion quality. Having the ability to change the fuser roller surface temperature within the time between consecutive receivers allows fusion and glossing to be tuned within a document run to receivers that are of different types, without reducing the productivity of the entire electrophotographic system.
Using radiant external surface heaters in sheet-fed processes presents the potential fire hazard of paper ignition. Radiant energy from a radiant external heater is not instantly dissipate when power is discontinued. The radiant surface will emit enough radiant energy to ignite a cellulose-base receiver such as paper if it has stalled under the radiant heater or wrapped around the fuser roller. The Xeikon DCP-32 color printer uses radiant heaters to heat its glossing rollers (GEM), allowing changes in the glossing rollers surface temperatures so that each receiver type will have consistent image gloss or quality. Although a web of paper rather than individual sheets is employed, there is still a substantial fire hazard if a safety system is not used. The Xeikon printer is provide with a safety system in the form of a clamshell that houses the radiant heat sources and closes if there are any jams that completely stop the machine. Closing of the clamshell isolates the radiant heat sources, preventing residual radiation from contacting the paper. Surrounding the clamshell is an enclosure housing comprising a nearly airtight insulated box, which greatly reduces the flow of air into the housing. The lack of oxygen would cause flames from a sheet of paper that had ignited in the radiant fusing system enclosure to die out quickly.
The present invention is directed a heater roller assembly for use with a fuser apparatus that comprises: a housing comprising a radiation shield enclosing a radiant heat source, and a heater roller that is disposed within the housing and is externally heated by the radiant heat source. The heater roller comprises: a core, a base cushion layer overlying the core, and an outer layer overlying the base cushion layer.
The outer layer of the heater roller comprises a fluorocarbon random copolymer having subunits of 
wherein x is from 10 to 90 mole %, y is from 10 to 90 mole %, x+y equal 100 mole %, and ORf is a perfluoroalkoxy group.
The present invention is further directed to a fuser apparatus that comprises a fuser roller whose outer surface is in contact with the outer layer of the heater roller included in the above described heater roller assembly. The fuser apparatus can further include a pressure roller that forms a nip with the fuser roller. In one embodiment of the apparatus of the invention, the pressure roller comprises a second fuser roller in contact with the outer layer of a heater roller of a second heater roller assembly.
Having the ability to quickly change the externally heated fuser roller surface temperature in accordance with the present invention allows for improved convenience in gloss control and thermal droop management The external radiant heating feature also allow, internal components to remain cooler than in an internally heated system, potentially resulting in increased component life.
An advantage of the present invention is that choosing a material such as a fluorocarbon random copolymer for the outer layer of the externally heated roller allows a large temperature gradient to be formed between the fuser roller and the heated roller, thereby maximizing heating time, or dwell time. Another advantage of the present invention is that the foam base cushion allows for compression to form a large pressure nip having virtually no velocity overdrive.